Ouabain (Oua) is a cardiac glycoside (M.W. 584.7) found in certain plant species such as the seeds of Strophanthus gratus (Jacobs, W. A., and Bigelow, N. M., J. Biol. Chem., 96:647-658 (1932)). Oua and ouabain-like compound (OLC) have also been found in humans and animals, including the hypothalamic inhibitory factor, HIF (Hamlyn, J. M., et al., Proc. Natl. Acad. Sci. USA, 88:6259-6263 (1991); Ludens, J. H., et al., J. Cardiovas. Pharm., 22:S38-S41 (1993); Tymiak, A. A., et al., Proc. Natl. Acad. Sci. USA, 90:8189-8193 (1993); U.S. Pat. No. 5,716,937; Haupert, G. T., Jr., and J. S. Sancho, Proc. Natl. Acad. Sci. USA, 76:4658-4660 (1979) and Haupert, G. T., Jr., et al., Am. J. Physiol., 247:F919-924 (1984)). While its function in plants is not known, in mammals Oua and/or OLC are believed to play a role in the regulation of sodium balance, arterial pressure and vascular smooth muscle tone under normal circumstances, and have a pathophysiologic role in common clinical disorders such as essential hypertension, pregnancy-induced hypertension, cardiac failure, salt sensitivity, chronic renal failure and cardiomyopathy (Goto, A. et al., Pharm. Reviews, 44:377-399 (1992); Manunta, P., et al., Hypertension, 34(3):450-456 (1999); and Blaustein, M. P., Kidney Internatl., 49:1748-1753 (1996)). The availability of specific molecular probes and reliable methods of detecting and measuring endogenous or exogenous Oua is the prerequisite to successfully investigating these issues.
A polyclonal antibody directed against ouabain exists, however, use of this antibody in diagnostic assays requires enrichment of the sample prior to contacting the sample with the antibody (Blaustein et al., U.S. Pat. No. 5,164,296). There is only one report in the literature of a mAb to Oua, but this Ab showed a high degree of cross-reactivity with digoxin (Dig), the cardiac glycoside in prevalent clinical use (Terano, Y., et al., Japan J. Med. Sci. Biol., 44:123-139 (1991)). Such an antibody would be of limited research and clinical use since many of the patients to be studied and treated for cardiovascular disease and renal disorders associated with ouabain and OLC, such as congestive heart failure and hypertension, are treated with digoxin. Thus, a need exists for improved probes, such as a more specific antibody, to detect and measure Oua and OLC and a method for measuring Oua and/or OLC wherein sample purification and/or enrichment is not necessary in the assay.